1. Field of the Invention
This invention relates to heat-responsive controls for gas or oil burning appliances and more particularly to fuel flow control devices for burners in hot water heaters, furnaces, boilers, stoves and the like. Specifically, this invention relates to a fuel flow safety control device for preventing continued overfiring of the burner.
2. Description of the Prior Art
A variety of fuel control devices have been utilized for years to control the flow of fuel such as gas or oil to the burners of various appliances such as hot water heaters, furnaces, boilers, stoves, and the like. In addition, temperature sensing safety devices have frequently been utilized in conjunction with such fuel control devices to prevent overheating of the appliance or detect pilot light failures.
Thermocouples are frequently utilized as temerature sensors in fuel control systems inasmuch as they generate thermoelectric currents at specified temperatures. One common use of thermocouples is to sense the pilot flame for a burner. In such a system, the electrical current generated by the thermocouple due to heat produced by the pilot flame will maintain the fuel control valve in an active state thereby allowing fuel to flow to the burner. If, however, the pilot flame is extinguished for any reason, the thermocouple will cease generating electricity thereby deactivating the fuel control valve and thus stopping the flow of fuel to the burner. In addition, it has been found desirable to connect various safety switches or devices in series with the thermocouple which are responsive to conditions within the appliance other than pilot flame failures, such as overheating of the appliance of overfiring by the burner.
U.S. Pat. No. 1,910,944, issued to F. D. Austin, is directed to a safety control apparatus of the above type which includes a thermally responsive circuit closure for the power circuit of an automatic fuel feed, and means for enabling the circuit closer to be subjected to internal boiler temperature while safeguarde from other internal boiler parameters. Also disclosed is a structure for preventng short circuiting of the power current incident to response of the circuit closure to excessive temerature. In particular, this patent provides for a fusible link positioned within a hot water boiler and connected in series with one of the power lines to the blower motor of the boiler. When a temperature develops within the boiler in excess of that normally tolerable, the fusible link will separate thereby shutting off power to the blower motor.
U.S. Pat. No. 2,788,417, issued to Fichter, is directed to heat-responsive controls for a fuel feeder to the burner of a furance. Associated therewith are means for breaking the feed under certain conditions, such as a gas control valve and a pilot light thermostat. These controls are shown to be connected in series as part of the electrical circuit leading from the power line and are intended to break the circuit through actuation of any one of the controls. The patent also teaches that when foreign matter collects at the electrical contacts of the thermostat, or when one of the movable elements has been distorted accidently or from tampering by inexperienced persons, the circuit is not broken when certain predetermined operating conditions have been reached and therefore will continue to operate and feed fuel to the burner. To prevent this from occurring, the patent also provides for a separate unit, having a fusible element, to be installed in any part of the mechanism. When the temperature at that location exceeds a predetermined limit, the fusible element will melt therby directly shutting off the power to the fuel control valve.
U.S. Pat. No. 3,924,099, issued to Housel, is directed to a forced circulation electric heater which includes a thermal fuse located between electrical heating coils and the fans and serves to deenergize the heating coils should excessive heat build-up occur in the housing. A thermal fuse block is illustrated in this patent to support a fusible element.
U.S. Pat. No. 3,652,195, issued to McIntosh et al, teaches a thermocouple control system which includes a thermocouple, an operator for a fuel control device, and a fusible link assembly connected in series with the thermocouple and the operator. The fusible link assembly, normally in a closed state, assumes an open state upon sensing a predetermined temperature. This deenergizes the operator by creating an open circuit, thereby shutting off the fuel flow to the burner. Specifically disclosed is a fusible link assembly located to the left and below the main burner of a furnace and connected in series with a thermocouple for pilot flame detection and an operator for a fuel control device such that if a sufficient temperature is built up in the lower part of the furnaces' combustion chamber sufficient to melt the fusible link, as caused by a reverse flow of air through the furnace flue, the safety valve in the fuel control device closes thereby breaking the fuel feed. However, the system disclosed in this particular patent requires the use of a very special fusible link assembly as well as a thermocouple adapted to sense a pilot flame. Thus, this device has the disadvantage of being relatively expensive and complex.